One thing discussed often and at great length in nerdy circles these days is this: Is the tablet—by which we really mean the Apple iPad, because it created the market, and presently accounts for nearly two thirds of it—a computer. A PC, if you will?

Some say that the iPad is not really a computer. It has no keyboard. People don’t sit at desks to use it. It lacks the horsepower of most of today’s computers. So they think it’s a big smartphone. I think they are wrong. They misunderstand what is happening.

This is not hard to see in action. Wandering around an airport cafe the other day, everyone had at least one device. But those with an iPad were by far the most comfortable, whether curled up in an armchair or sitting at a table. And they were doing everything: I saw one guy watching a movie, another writing a letter, another CEO-type playing Angry Birds. I was thrown out of the cafe before I was able to finish my research.

At the hairdressers no fashion magazines were being read: Everyone was cradling an iPad, oblivious to the time and their hair being teased into odd shapes.

So let’s look at the data.

Surveys by comScore, a metrics company, point to what is really happening. In studies in the U.S. last October and of Europe released this week [Registration required], they noticed that during the week tablet usage spikes at night—as computer usage drops off. So while during the work day folk are using their PCs, come evening they switch to tablets. (Mobile usage, however, remains flat from about 6 pm.)  The drop in PC usage is even more pronounced in the U.S., while tablet usage in the evening continues to rise until about 11 pm:

In other words, people are using their tablets as computers. Not as mobile devices. Not as replacements for their phone. They’re using them, in the words of a friend of mine, as a replacement to that ancient computer sitting in the corner gathering dust that gets booted up once in a while to write an email or a letter to Granny on.

Now not everyone is using tablets like this. The first surveys of tablet usage indicated they were using them as ‘TV buddies’—things to play with while watching TV. But this still doesn’t quite capture what is happening.

One study by Nielsen found last May that 3 out of 10 Americans were using their computer less frequently after buying a tablet. What’s surprising about this figure is that it’s higher than for all other devices—including gaming consoles, Internet-connected TVs and portable media players. Given the plethora of games and stuff you can get for a tablet, surely more people would be saying that they use these devices less than their netbook, laptop or desktop, now they have a tablet?

That survey was done when less than 5% of U.S. consumers owned one. A year on, that figure is much higher. Pew’s Internet and American Life Project reported on Jan 23 that the number of American adults who owned a tablet grew from 10% to 19% over the holiday period; although their data may not be directly comparable with Nielsen’s it sounds about right. And represents an unprecedented adoption of a new device, or computing platform, or whatever you want to call it.

(Pew also surveys ebook readers and finds the same results. But I think we’ll see a serious divergence between these two types of device. Yes, some tablets are good for reading and some ereaders, like the Kindle Fire, look a lot like a tablet. But they’re different, and used in different ways. I think that while the market will overlap even more, they’ll be like more like the laptop and netbook markets, or the ultrabook and the PC market: they may do similar things but the way people use them, and the reason people buy them, will differ.)

This is rapidly altering the demographics of the average tablet user. Back in 2010, a few months after the first iPad was launched, 18-34 year olds accounted for nearly half the market, according to another Nielsen report. A year on, that figure was down to a little over a third, as older folk jumped aboard. Indeed the number of 55+ iPad users doubled in that period, accounting for more than 25-34 year old users.

(Pew’s figures suggest that while older folk have been slower to adopt, the rate of growth is picking up. Around a quarter of adults up to the age of 49 now have a tablet in the U.S. (a shocking enough figure in itself.) Above 50 the number comes down. But the telling thing to me is that the rate of growth is more or less the same: about a fourfold growth between November 2010 and January 2012. While a lot of these may have been gifts over the holidays, it also suggests that the potential is there.)

So it’s pretty simple. The tablet, OK, the iPad came along and reinvented something that we thought no one wanted—a tablet device with no keyboard. But Apple’s design and marketing savvy, and the ecosystem of apps and peripherals, have made the tablet sexy again. Indeed, it has helped revive several industries that looked dead: the wireless keyboard, for example. ThinkOutside was a company in the early 2000s that made wonderful foldable keyboards for the Palm, but couldn’t make it profitable (and is now part of an apparently moribund company called iGo).

Now look: the website of Logitech, a major peripherals company, has the external keyboard and stand for the iPad as more or less its top product. Logitech reckon a quarter of tablet users want an external keyboard, and three quarters of them want their tablet “to be as productive as their laptop.” Most peripheral companies offer a kind of wireless keyboard, and there are more on the horizon.

And as BusinessWeek reported, the highest grossing app on the iPad appstore this Christmas wasn’t Angry Birds; it was a program for viewing and editing Microsoft Office documents, called QuickOffice. The app itself is not new: it’s been around since 2002, and a paired-down version came preinstalled on dozens of devices. But people wouldn’t shell out the extra $10 for the full version—until the iPad came along. Now they happily pay $20 and the company sold $30 million’s worth in 2011. (BusinessWeek links this to growing corporate interest in the iPad but you can see from comScore’s data that this is not necessarily correct. The tablet is a personal device that is mostly used outside the office.)

So. There’s a new industry out there, and it’s for a device that’s not a phone, though it has the same degree of connectivity; it’s not a desktop, though it should be able to do all the things a desktop can do; it’s not a laptop, though it should make the user as productive as a laptop can. And it’s many more things besides: a TV buddy, a sort of device to accompany your downtime in cafes, salons or on the couch.

Gartner, a research company, reckon that from about 17.5 million devices sold in 2010 there will be 325 million sold in 2015. An 18-fold increase. In the same period the annual sales of notebooks will only have doubled, and desktops will have grown by, er, 5%. Hard not to conclude from that that the tablet, OK, the iPad, is going to be everyone’s favorite computer—replacing the desktop, the laptop and whatever ultrabooks, netbooks or thinkbooks are the big thing in 2015.

(Update: This was written before Apple’s results. Tim Cook has confirmed the PC is their main competitor.)